Fuel cells are well known and are commonly used to produce electrical current from a hydrogen-rich fuel stream and an oxygen-containing oxidant stream to power electrical apparatus. Fuel cells are typically arranged in a cell stack assembly having a plurality of fuel cells arranged with common manifolds and other components such as controllers and valves, etc. to form a fuel cell power plant. Many such power plants utilize a “membrane electrode assembly” (“MEA”) that includes a “proton exchange membrane” (“PEM”) as an electrolyte secured between opposed anode and cathode catalysts and support materials.
In such a fuel cell power plant of the prior art, it is well known that many difficulties are associated with long-term operation of the plant. In particular, fuel cell power plants that include a coolant system that directs a coolant fluid through a sealed coolant flow field in thermal exchange with the MEA to remove heat generated during operation of the fuel cells must carefully control a relative humidity of reactant gas streams passing adjacent the MEA indirectly by controlling the temperature of the reactant gases. (For purposes herein a “sealed coolant flow field is to mean that fluids cannot pass between the coolant flow field and adjacent fuel cell components.) If the relative humidity is too high, water generated at the cathode catalyst during operation of the cell will accumulate as a liquid instead of evaporating into the reactant stream. This produces flooding which slows down or completely interrupts flow of the reactant stream and results in poor or disrupted fuel cell operation.
In contrast if the relative humidity of the reactant streams is too low, moisture within the PEM electrolyte within the MEA will evaporate into the reactant streams resulting in drying of the PEM. This slows transfer of protons through the PEM which in turns interferes with electricity production. Drying of the PEM also results in deterioration of the PEM so that gaseous reactant breakthrough of torn or disrupted membranes is possible. This not only deteriorates fuel cell performance, but also poses a risk of mixing of reactant gases that could lead to combustion of the gasses.
Consequently, there is a need for a fuel cell power plant having sealed coolant flow fields that efficiently maintains relative humidity of reactant streams passing through fuel cells of the plant.